Shadow + border effect

Is the a simple way to achive the following look & feel for tab headers:
(Specifically the tab-border and the shadow):

There is not a simple way. All skins, except dhx_web, use images to for tabs:

dhtmlxTabbar/samples/04_styling/01_skins.html
So, you will need to create 3 background images for active tab and 3 for inactive tabs.

Regarding shadow - you may set box-shadow for in the dhx_tablist_line class to set shadow for the top part of content area:

.dhx_tablist_line{
/box-shadow definition/
}

The style of the container for the active tab is defined by dhx_tab_element_active class.

Started to look at it, it looks as if the style of the tab is not only the CSS code, but is also hard coded. For instance, the usage of these images on active/disabled, is such that specific style properties are being applied instead of just changing the className to an appropriate class, right?

For instance, the usage of these images on active/disabled, is such that specific style properties are being applied instead of just changing the className to an appropriate class, right?

Yes, the path to background image is set in the dhtmlxtabbar.js as well as background position.

the path to tab image consists of imagePath+skinName+"/"+imageName